Light weight steel and its use for car parts and facade linings

ABSTRACT

A high-strength lightweight steel and its use for car parts and facade linings is a purely ferritic steel having, in mass %, more than 5 to 9% Al, less than 0.2% Si, and 0.03 to 0.2% Mn.

BACKGROUND OF THE INVENTION

The invention relates to a high-strength lightweight steel and its usefor car parts and facade linings.

All of the content indications below are in percentage of mass.

High-strength construction steel types have been developed for thevehicle industry with different properties and have already been used inproduction to a significant extent. A reduction in weight as comparedwith conventional soft steel can be obtained due to a reduction of sheetthickness thanks to greater strength. To ensure sufficient corrosionresistance, different surface coating processes were developed(Stahl-Eisen-Werkstoffblatt SEW 094 and SEW 093; Stahl und Eisen 106(1986), No. 12, pages 21-38 and 114 (1994), No. 7, pages 47-53).

Steel types with greater aluminum content are known. Thus, EP-A-0 495121 discloses steel with up to 7% Al, more than 0.5% Si, 0.1 to 8% Mnand less than 0.01% C, N, O, P for an attenuation of vibration andnoise.

EP-A-0 401 098 takes steel types into account with less than 3.3% Si and1.5 and 8% Al for soft magnetic sheets with a sharp (100) (001) texture(cube layer). Interstitial impurities must be below 50 ppm, C below 30ppm. The texture used is unsuitable for deformation processes such asdeep drawing and stretch forming.

DE 43 03 316 A describes steel types with 13 to 16% Al and in partlarger contents in other alloy elements (Cr, Nb, Ta, W, Si, B, Ti) foroxidation and corrosion resistant parts.

DE 32 01 816 A indicates alloys with 1 to 10% Al for parts which comeinto contact with hydrocarbon-containing liquids at high temperatures(in the range of 750 to 900° C.) so that no hydrocarbon deposits occur.The surface of the parts can be pre-oxidized.

The above-described state of the art has the following disadvantages:

Weight reduction can only be achieved by reducing the sheet thickness orthrough additional constructive and/or joining measures;

The required corrosion protection can only be achieved by providingadditional surface coatings.

Deep-drawn steel types containing greater amounts of aluminum which canbe well formed or deep-drawn and stretch-drawn, cold rolled and annealedwith re-crystallization, such as are needed in the automotive technologyor as facade linings, are not part of the state of the art.

It is therefore the object of the present invention to create a steelwith a density significantly below 7.6 g/cm³, with greater strength andgood cold formability and at the same time better resistance toatmospheric corrosion than conventional deep-drawn steel.

SUMMARY OF THE INVENTION

The purely ferritic steel according to the invention is characterized bymore than 5 to 9% Al, <0.2% Si, 0.03 to 0.2% Mn, the remainder iron andimpurities caused by melting, including up to a maximum of 1% in all ofCu+Mo+W+Co+Cr+Ni and a maximum of 0.1% in all of Sc+Y+rare earths. Itmay contain in addition

up to 0.1% C

up to 0.5% in all of Ti+Zr+Hf+V+Nb+Ta

up to 0.01% B

up to 0.1% P.

The aluminum content is preferably in a range between 7 and 9%.Furthermore the steel may be alloyed with titanium and/or niobium of atleast 0.03%.

The following are special characteristics of the steel composition:

the steel according to the invention is purely ferritic;

it is limited to a maximum of 0.2% content in Si

it contains a small amount of carbon, below 0.1% and noalloy-significant content in Cu, Mo, W, Co, Cr, Ni, Se Y and rare earthmetals.

The steel according to the invention possesses an unexpectedly goodcombination of previously unknown, advantageous characteristics whichcan be described as follows:

Strength characteristics are markedly higher than those of conventional,soft deep-drawn steel;

Formability, as measured against strength, is comparatively good;

Density is distinctly lower than that of conventional deep-drawn steeltypes;

Corrosion resistance is considerably improved.

The deep-drawable and stretch-drawable steel with higher content inaluminum is melted down, is poured into a billet, rolled within atemperature range above re-crystallization temperature, or is poured offin form of a band. The steel is either processed directly as a hot bandor is cold rolled after hot rolling, with a degree of deformation ofmore than 20%. The cold band is then re-crystallized and annealed.

Due to its good cold formability and low density, clearly below 7.6g/cm³, the steel in form of sheets is especially well suited forapplications in the automotive industry and as facade linings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail below through the exampleof embodiments shown in the drawings wherein:

FIG. 1 is a graph showing cyclic current-density—potential curves of aninventive iron-aluminum alloy compared with pure iron;

FIG. 2 is a graph showing cyclic current-density—potential curves of aninventive iron-aluminum alloy with various electrolytic or thermalafter-treatments;

FIG. 3 is a graph showing weight reduction of inventive alloys incomparison with conventional deep-drawn steels; and

FIG. 4 is a bar graph showing the increase in aluminum content at thesurface of inventive alloys.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLES

The initial material was melted down in a vacuum induction oven andpoured into casting dies. Hot rolling was carried out at a temperatureranging from 800° C. to 1100° C. on thicknesses of 4 mm. Followingpickling, tablets between 5 and 92% were cold-rolled and thenre-crystallized and annealed between 700° C. and 900° C. Table 1indicates the chemical composition of several examined steel types.

TABLE 1 Chemical composition In weight %, C, N, O in ppm Steel C Si Mn PS Al N O Nb 1 220 0.024 0.031 0.006 0.002 5.1 10 n.b. — 2 130 0.0240.034 0.008 0.002 7.0 15 n.b. — 3  60 0.029 0.032 0.007 0.002 8.8 14n.b. — 4  39 0.01 0.10 0.008 n.b. 5.4 10 n.b. — 5  39 0.01 0.12 n.b.n.b. 7.9  8 34 — 6  36 0.01 0.14 n.b. n.b. 9.0  5 n.b. — 7 260 0.04 0.190.008 0.003 5.1 25 n.b. — 8 270 0.08 0.19 0.012 0.003 7.8 24 n.b. — 9100 n.b. n.b. n.b. n.b. 7.4 16 20  0.05 10 100 n.b. n.b. n.b. n.b. 7.416 19 0.1 11 100 n.b. n.b. n.b. n.b. 7.4 16 19 0.2 12 100 n.b. n.b. n.b.n.b. 7.4 16 18 0.4

Table 2 shows the strength and formability characteristics of severalexamined types after 70% deformation in the annealed, re-crystallizingstate. In this table:

R_(p)=stretching limit

R_(m)=tensile strength

A80=extension, rod length 1=80 mm

E=elasticity module

rL=r value (anisotropy value) in longitudinal sense

n=n value (hardening index)

TABLE 2 Strength and formability characteristics in longitudinal senseSteel R_(p) (MPa) R_(m) (MPa) A80 (%) E (GPa) rL value n value 1 340 44028 190 0.79 0.195 2 390 490 28 180 0.73 0.175 3 440 540 n.b. 170 0.580.130 4 330 470 29 180 0.83 0.250 5 420 550 27 180 0.88 0.177 6 460 510n.b. 170 n.b. n.b. 7 380 470 25 190 n.b. n.b. 8 480 570 22 180 n.b. n.b.9 400 490 25 n.b. n.b. n.b. 10  310 450 30 n.b. n.b. n.b. 11  300 460 24n.b. n.b. n.b. 12  310 470 31 n.b. n.b. n.b.

Table 3 documents good strength and formability characteristics forsamples in cold-rolled and annealed state, as well as for hot-rolledsamples, among others A5—elongation at rupture 1=5 d.

TABLE 3 Strength and Formability Characteristics in Transversal SenseSteel R_(p) (MPa) R_(m) (MPa) A5 (%) E (GPa) n 1 350 480 22 200 018 2460 580 20 190 0.15 3 560 650 n.b. 180 n.b 4 330 460 29 200 0.18 5 390510 27 190 0.16 6 480 550 n.b. 170 n.b.

Table 4 shows the influence of the cold rolling degree KVG in % offorming characteristics. It can be seen that as the cold rolling degreeincreases up to 70%, the r and n values increase significantly.

TABLE 4 r and n value of the re-crystallizingly annealed steel 4 infunction of the cold rolling degree KVG in % KVG 5 10 15 20 30 50 70 92rL 0.7 0.56 n.b. 0.61 0.72 0.77 0.80 0.42 n 0.16 0.16 0.16 0.16 0.170.175 0.195 0.19

Table 5 shows the results of Erichsen swaging tests according to DIN50101, which were conducted to determine formability characteristicsrelevant to practical applications.

TABLE 5 Erichsen swaging test (stamp diameter + 20 mm) of there-crystallizingly annealed steel types Steel Sheet thickness in mmSwaging in mm 1 0.98 9.6 1 0.96 10.0 1 0.97 9.5 4 1.10 9.7 4 1.10 9.9

FIG. 1 shows cyclic current-density - potential curves of iron-aluminumalloys by comparison with pure iron. An iron-aluminum alloy with apolished surface, i.e. without a protective oxide layer, possessesalready a better corrosion resistance than pure iron. Throughelectrolytic bonification of the surface with aluminum, the goodcorrosion properties of iron-aluminum alloys can be further improved.

FIG. 2 shows that an electrolytic bonification with subsequent thermaltreatment as compared with an alloy with polished surface, i.e. withoutprotective oxide layer, tight and corrosion-proof surface layers can beproduced in a very short time.

In FIG. 3 the weight reduction of iron-aluminum alloys is entered as afunction of the aluminum content. It becomes clear that with an aluminumcontent in the claimed range of 5 to 9% in the steel according to theinvention, a weight reduction of 4.5 to 12% can be achieved.

Due to the strongly mixed crystal solidifying effect of aluminum iniron-aluminum alloys, and due to the presence of elements accompanyingsteel and micro alloy elements, the strength increases considerably ascompared to micro-alloyed thin-sheet steel. In addition to good strengthand formability characteristics accompanied by distinct weightreduction, the steel according to the invention is distinguished bygreater resistance to corrosion. This can be improved even furtherthrough a chemical, electrochemical or thermal treatment, when theformation of an aluminum-rich surface layer results in the production ofa protective A1₂O₃ covering layer.

Table 6 shows the increase of the aluminum content at the surface of aniron alloy with 8.5% Al improved on the surface with Al by electrolyticafter-treatment at 20 and 60° C. in the active (−0.17 V against NHE),passive (1.1 V against NHE) and transpassive (10.65 V against NHE)range. In a comparison with the non-treated alloy an increase inaluminum concentration at the surface by almost 100% resulted. Identicalresults can also be achieved through electrochemical after-treatmentwith Al.

TABLE 6 Surface layer on an FeAl 8.5 alloy produced by electrolyticalafter-treatment, with Al bonification Increase Al/(Al + Fe) in At. % ofAl at the surface in % Polarization 20° C. 60° C. 20° C. 60° C. active21.1 19.3 28.7 17.7 passive 18.9 29.4 15.2 79.3 transpassive 29.9 32.382.3 96.3 polished sample 16.4

FIG. 4 is a bar graph which plots the values listed in Table 6.

Dense Al₂O₃ layers can be constituted through suitable thermalafter-treatment at higher temperature (600 to 1200° C.).

What is claimed is:
 1. Vehicle parts and facade linings made fromhigh-strength lightweight steel comprising (in mass %): more than 5 to9% Al, <0.2% Si, 0.03 to 0.2% Mn, remainder iron and impurities causedby melting including a maximum of 1% of Cu+Mo+W+Co+Cr+Ni and up to 0.1%of Sc+Y+rare earth metals.
 2. The vehicle parts and facade linings ofclaim 1 further comprising (in mass %): a maximum of 0.1% C; a maximumof 0.5% of Ti+Zr+Hf+V+Nb+Ta; a maximum of 0.01% B; and a maximum of 0.1%P.
 3. The vehicle parts and facade linings of claim 2, wherein the totalcontent of titanium and niobium is at least 0.03%.
 4. The vehicle partsand facade linings of claim 1, wherein the content of Al is in the rangeof 7 to 9%.
 5. The vehicle parts and facade linings of claim 1, furthercomprising bands provided with a chemical, electrochemical, organicnon-metallic or metallic coating.
 6. The vehicle parts and facadelinings of claim 5 wherein the bands are coated with aluminum.